The present disclosure relates to an image reading apparatus that irradiates a document by a side light method of guiding light emitted from an LED in a main scanning direction by a light guiding body to thereby read an image of the document, and an image forming apparatus such as a copy machine provided with the image reading apparatus.
An image forming apparatus, such as a copying machine or the like that forms an image on paper using an electrographic method, includes an image reading apparatus configured to optically read a document image. In recent years, an LED is used as a light source for the image reading apparatus in substitution for a conventional xenon lamp or a cold cathode lamp.
An image reading apparatus using an LED as a light source includes a type that uses a side-light method in which light emitted from an LED disposed on one end in the main scanning direction is introduced into the main scanning direction by a light guiding body. FIGS. 5 and 6 illustrate the configuration of an illumination unit of a conventional image forming apparatus employing a side-light method.
FIG. 5 is a side view of the illumination unit of the image reading apparatus employing a side-light method. FIG. 6 is a front view of the illumination unit. As shown in FIG. 5, in the image reading apparatus employing a side-light method (image forming apparatus), an LED 106 as a light source is disposed on one end in the main scanning direction (the transverse direction (lateral direction) in FIG. 5), and a light guiding body 107 is disposed in the vicinity of the LED 106 along the main scanning direction. An incident surface 107a of one end in the main scanning direction of the light guiding body 107 faces the LED 106. In addition, prisms 121 that include a plurality of V-shaped grooves on a lower surface are disposed on the opposite side to an emission surface 107b on the upper surface of the light guiding body 107 at a suitable interval in the main scanning direction.
A reflector 122 is arranged below the light guiding body 107 along the main scanning direction. A platen glass 123 is provided above the light guiding body 107 to mount the document.
In such an image reading apparatus, when the LED 106 is illuminated, the light is incident from the incident surface 107a of the light guiding body 107 into the light guiding body 107 as illustrated in FIG. 5, and is reflected by an inner surface of the light guiding body 107 and propagated in the main scanning direction. A part of the incident light is reflected by the incident plane of the prism 121, is emitted from the emission surface 107b through the platen glass 123, and illuminates a document that is mounted on the platen glass 123. The light that is not fully reflected by the inner surface of the light guiding body 107 and emitted from an anti-incident surface (lower surface) of the light guiding body 107 is reflected by the reflector 122 and incident again onto the light guiding body 107. Light energy of such light is significantly lost due to Fresnel loss upon reflection by the reflector 122 and second incidence onto the light guiding body 107.
In addition, conventionally, height, pitch and the like of the prisms 121 are set such that illuminance of the light illuminating the document is uniform in the main scanning direction. For example, in a case in which the height of the prisms 121 is constant, the pitch of the prisms is set to be greater in the vicinity of the LED 106 and to decrease as the distance from the LED 106 increases.
On the other hand, in a case in which the pitch of the prisms 121 is constant, the height of the prisms is set to be smaller in the vicinity of the LED 106 and to increase as the distance from the LED 106 increases.
The light that is emitted from the LED 106 and incident upon the light guiding body 107 includes: direct light that is reflected once by the inner surface of the light guiding body 107 and then incident upon the prisms 121; and indirect light that is reflected at least twice by the inner surface of the light guiding body 107 and then incident upon the prisms 121. In the vicinity of the LED 106, the proportion of the direct light is greater than that of the indirect light. And, as the distance from the LED 106 increases, the proportion of the indirect light increases compared to that of the direct light.
Here, as the direct light has a short incident distance to a diffusion region and is high in energy, the illuminance of a region in the vicinity of the LED 106 is high and an illuminance distribution in a vertical scanning direction becomes sharp.
FIG. 7 shows energy distribution (light intensity density) of the direct light and the indirect light in the main scanning direction; and FIG. 8 shows cross-sectional distributed light in the vertical scanning direction in the vicinity of the LED and in a separated region located away from the LED.
As shown in FIG. 8, the cross-sectional distributed light in the vertical scanning direction in the vicinity of the LED is sharply distributed in a steep and narrow mountain shape. Given this, if the document is spaced away from the platen glass in the vicinity of the LED 106, the light intensity at the part of the document is significantly reduced and image fogging may be caused due to variation in image density.
As a related art to the image reading apparatus employing the side-light method, proposed is a configuration of arranging a central position of the LED out of alignment with a normal line in a center of width in a lateral direction of the diffusion region, in order to uniformly illuminate a whole document illumination position along the main scanning direction. According to the related art, the direct light that is directly incident from the LED upon the diffusion region is reduced and the indirect light that is reflected by the inner surface of the light guiding body except for the diffusion region can be increased, thereby allowing the light to propagate from the LED to a position away therefrom, and improving unevenness of illuminance in the main scanning direction.
However, in the configuration of the related art, there is significant influence of the direct light in the vicinity of the LED and it is difficult to broaden the distributed light in the vertical scanning direction on the surface of the document, leading to a problem of variation in image density due to the document being spaced away from the platen glass.